1. Field of the Invention
The invention relates to a cylinder head of an internal combustion engine, and more particularly to a cylinder head having a cylinder head body, a camshaft housing mounted on the cylinder head body, and cam caps mounted on the camshaft housing.
2. Description of the Related Art
Japanese Patent Application Publication No. 2004-92567 (JP-A-2004-92567) recites a cylinder block of an internal combustion engine having a cylinder head body mounted on the cylinder block of the engine, a camshaft housing mounted on the cylinder head body, and cam caps mounted on the camshaft housing. The camshaft housing has bearing portions for supporting the camshafts of the engine. The camshafts are rotatably supported by the bearing portions of the camshaft housing and the cam caps.
The reason for employing such a structure in which the cylinder head and the camshaft housing are separately provided is to improve the workability of mounting engine components on the engine. More specifically, on one side, the engine valves and some components of the valve drive mechanism are mounted to the cylinder head body, and on the other side, the camshafts are mounted to the bearing portions of the camshaft housing. Then, the camshaft housing is secured to the cylinder head body by several bolts, whereby the mounting of the valve drive mechanism to the cylinder head is completed. According to this method, thus, the workability of mounting the engine components to the engine improves.
Meanwhile, in order to rotatably support the camshafts, circular cam holes are formed between the camshaft housing and the respective cam caps. More specifically, the cam holes are formed by semi-circular bearing concave portions formed on the respective bearing portions of the camshaft housing and semi-circular bearing concave portions formed on the respective cam caps and facing the corresponding semi-circular bearing concave portions of the bearing portions of the camshaft housing. Each of these cam holes is required to have a predetermined level of roundness. Therefore, as mentioned above, it is necessary to round each cam hole before mounting the camshafts.
The rounding of the cam holes needs to be performed in a state that is the same as or similar to when the camshaft housing and the cam caps are actually mounted on the engine. That is, when the bolting points for bolting the camshaft housing and the cam caps and the tightening axial force for tightening each bolt are different from those when mounting the camshaft housing and the cam caps to the engine, even if the cam holes are rounded to achieve the predetermined level of roundness, the roundness of each cam hole may be reduced when the camshaft housing and the cam caps are mounted to the engine. Therefore, conventionally, the rounding of the cam holes is performed after mounting the camshaft housing and the cam caps to the cylinder head body or a dummy cylinder head body.
In this case, however, after rounding the cam holes, it is necessary to remove the camshaft housing and the cam caps from the cylinder head body or a dummy cylinder head body and then mount them again to the cylinder head body when assembling the engine. Thus, according to such a conventional cam hole rounding process, because the cylinder head body or a dummy cylinder head body is used when rounding the cam holes, the rounding process is costly and requires much work despite the fact that the camshaft housing and the cylinder head body are separately provided. Further, when a dummy cylinder head body is used to round the cam holes, the cost for the dummy cylinder head body increases the production cost.